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use constant FOO => 42;

sub FOO { 42 }

use POSIX ();

$ perl -mPOSIX -e '$a = POSIX::O_RDWR; print "$a\n"'
POSIX::O_RDWR
$ perl -mPOSIX -e '$a = &POSIX::O_RDWR; print "$a\n"'
2

use POSIX qw(O_RDWR);

$ perl -mPOSIX=O_RDWR -e '$a = O_RDWR; print "$a\n"'
2